On Sun, May 31, 2026 at 4:50 AM 신성준 <[email protected]> wrote:
> Hi hackers, > > The write(2) calls that flush server log output aren't covered by wait > events. When a backend logs something, the writes go out in: > > - write_pipe_chunks(): write(2) to the syslogger pipe > - write_console(): write(2) to stderr (WriteConsoleW() on Windows) > > If one of those blocks -- syslogger pipe full, slow console, slow log > device -- pg_stat_activity just shows wait_event = NULL until it > returns. Since NULL usually reads as "on CPU", a backend stuck writing > logs looks like it's doing work, so logging-related stalls are easy to > miss. > > Attached is a short series that adds two WaitEventIO events and reports > them around those writes: > > IO / SysloggerWrite - write(2) to the syslogger pipe > IO / StderrWrite - write(2) to stderr, and WriteConsoleW() > > 0001 adds the events and covers the write(2) paths. 0002 does the > Windows WriteConsoleW() path, split out since it's platform-specific. > > It only wraps the leaf write call and uses the existing > pgstat_report_wait_start()/end() helpers, so it stays allocation-free > and safe to call from inside the error-reporting path. > > I did a quick before/after to make sure the events show up: 8 backends > each emitting large RAISE LOG lines, sampling wait_event from > pg_stat_activity every 50 ms for 20 s. > > - logging_collector = on (syslogger pipe): > master: NULL 100.0% (2184/2184) > patched: IO/SysloggerWrite 99.1% (2204/2224), NULL 0.9% > > - logging_collector = off (stderr): > master: NULL 100.0% (2144/2144) > patched: IO/StderrWrite 90.7% (1952/2152), NULL 9.3% > > On master that wait time is just invisible; with the patch it lands on > the new events. I can send the scripts and raw samples if anyone wants > to reproduce it. > +1 Nice. We have too many waits that are registered as CPU. > >
